1. Field of the Invention
The present disclosure is directed in general to security systems and in particular to a security system that includes a transmitter for operating a rolling code receiver.
2. Description of the Related Art
Transmitter-receiver controller systems are widely used for remote control and/or actuation of devices or appliances such as garage door openers, gate openers, and security systems. Rather than transmitting a single code N to operate the receiver, rolling code technology is based on the idea that the recognized operating code of the security system changes each time an operating code is provided. The activation code is altered each time in both the transmitter and the receiver according to a rolling code algorithm, which produces a specific number of possible code combinations. In most cases, the transmitter and receiver of a rolling code system both contain a synchronized code generator that calculates a new operating code each time a code is provided and/or received. Thus, the operating code combination N of the system changes to code combination N+1 after code N is used, then code N+1 changes to code combination N+2 and so on.
In the case of a transmitter, its code generator produces a new code (e.g., N+1) each time it transmits a code, whether or not the receiver actually received the new code. While in the case of the receiver, its code generator advances to a new code (e.g., N+1) only when it receives a valid code. However, where the transmitter transmits a code, but the receiver does not receive the transmitted code, the transmitter and receiver will be out of synchronization. That is, the code generator in the transmitter will be further along in the code sequence than the code generator in the receiver. This may occur, for example, when the transmitter is activated outside the maximum range of the receiver. Thus, when a rolling code transmitter is activated “out of range,” the transmitter will transmit code N and advance its rolling code to code N+1, but the receiver will remain at code N and continue to expect code N. When the rolling code transmitter is activated “in range,” it will transmit code N+1, but the rolling code receiver will not respond because it expects code N.
To avoid having to reset the rolling code generator each time the transmitter and receiver are out of synchronization, manufacturers of rolling code systems provide code windows. Some manufacturers provide one or more forward windows, while others will also provide a backward window. Rolling code receivers having code window will be activated, not only by the current code N in the rolling code sequence, but also at any other code in the designated code window.